Convertible Debt and Notes Payable (Details)			3 Months Ended		12 Months Ended	40 Months Ended
		Dec. 05, 2016	Mar. 31, 2018 USD ($) letters_of_credit	Mar. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Jan. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)	Dec. 14, 2017 USD ($) $ / shares	Dec. 08, 2017 USD ($)	Jan. 31, 2016	Mar. 10, 2014 USD ($) $ / shares
Debt and Capital Lease Obligations [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Components of short-term borrowings and other debt
Convertible debt			$ 142,962,000				$ 141,502,000
Note payable			133,125,000				132,970,000
Line of Credit, Current			85,678,000				0
Loans Payable to Bank			28,161,000				28,121,000
Other Notes Payable			9,332,000				8,247,000
Capital lease obligations			3,948,000				4,120,000
Debt, Long-term and Short-term, Combined Amount			403,206,000				314,960,000
Amortization of debt discount			1,442,000	$ 1,840,000
Purchase of treasury stock, cost			7,584,000		$ 300,000
Letter of credit, borrowing capacity			180,000,000
Capital Lease Obligations Incurred						$ 7,300,000
Capital Leases, Income Statement, Interest Expense			100,000
Future minimum lease payments for capital lease obligations
2017			1,562,000
2018			802,000
2019			724,000
2020			724,000
2021			491,000
Thereafter			0
Subtotal			4,303,000
Less: Amount representing interest	[1]		(355,000)
Capital lease obligations			$ 3,948,000				4,120,000
Number of Letters of Credit | letters_of_credit			8
Restricted Cash and Cash Equivalents			$ 14,786,000	14,696,000			17,888,000
Minimum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term			48 months
Capital leases, interest rate			3.70%
Maximum
Components of short-term borrowings and other debt
Capital Lease Obligation, Initial Term			60 months
Capital leases, interest rate			5.69%
Letter of Credit, San Francisco Office, Expires January 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			$ 711,934
Letter of Credit, Connecticut Office, Expires September 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			65,425
Letter of Credit, Boston Office, Expires March 2019 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			384,958
Letter of Credit, NY Office 1, Expires April 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			355,950
Letter of Credit, NY Office 2, Expires May 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			70,142
Letter of Credit, NY Office 3, Expires October 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			496,418
Letter of Credit, NY Office 4, Expires October 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			2,248,893
Letter of Credit, NY Office 5, Expires November 2018 | Letter of Credit
Components of short-term borrowings and other debt
Letter of credit, borrowing capacity			1,610,750
Convertible Debt
Components of short-term borrowings and other debt
Convertible debt, unamortized discount			25,800,000				25,600,000
Repayments on long-term and short-term borrowings
2017			4,577,000
2018			38,925,000
2019			4,050,000
2020			4,050,000
2021			139,050,000
Thereafter			0
Subtotal			190,652,000
Less: Amount representing interest	[1]		(47,690,000)
Senior Notes
Components of short-term borrowings and other debt
Note payable			133,125,000
Repayments on long-term and short-term borrowings
2017			7,607,000
2018			10,143,000
2019			10,143,000
2020			10,143,000
2021			10,143,000
Thereafter			188,715,000
Subtotal			236,894,000
Less: Amount representing interest	[1]		$ (103,769,000)
Notes Payable to Banks [Member]
Components of short-term borrowings and other debt
Short-term Debt, Percentage Bearing Variable Interest Rate			3.75%
Debt Instrument, Face Amount			$ 28,200,000.0
Interest Expense, Debt			400,000
Insurance Note [Member]
Components of short-term borrowings and other debt
Other Notes Payable			1,500,000
Interest rate										2.01%
Debt Instrument, Periodic Payment, Principal			200,000
Short-term Debt, Maximum Amount Outstanding During Period			2,000,000
Loans
Repayments on long-term and short-term borrowings
2017			28,551,000
2018			0
2019			0
2020			0
2021			0
Thereafter			0
Subtotal			28,551,000
Less: Amount representing interest	[1]		(390,000)
Insurance Note and Aircraft [Member]
Repayments on long-term and short-term borrowings
2017			3,023,000
2018			3,410,000
2019			1,363,000
2020			2,512,000
2021			0
Thereafter			0
Subtotal			10,308,000
Less: Amount representing interest	[1]		(976,000)
Aircraft and related equipment
Components of short-term borrowings and other debt
Other Notes Payable			7,800,000				8,200,000
Interest Expense, Debt			100,000	200,000
Aircraft and related equipment | Minimum
Components of short-term borrowings and other debt
Interest rate										4.21%
Aircraft and related equipment | Maximum
Components of short-term borrowings and other debt
Interest rate										7.25%
2022 convertible note [Member] | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			110,700,000
Interest on Convertible Debt, Net of Tax			1,000,000
Debt Instrument, Convertible, Conversion Price | $ / shares								$ 17.375
Convertible debt, unamortized discount											$ 23,400,000
Amortization of debt discount			1,000,000
Debt Instrument, Interest Rate, Effective Percentage											7.13%
Debt Issuance Costs, Gross											$ 2,100,000
Debt Instrument, Face Amount								$ 135,000,000.0
2019 convertible note [Member]
Components of short-term borrowings and other debt
Purchase of treasury stock, cost			19,500,000
2019 convertible note [Member] | Convertible Debt
Components of short-term borrowings and other debt
Convertible debt			32,300,000
Interest rate											3.00%
Principal amount of notes being converted											$ 1,000
Interest on Convertible Debt, Net of Tax			500,000	1,100,000
Debt Instrument, Convertible, Conversion Price | $ / shares											$ 21.32
Debt Instrument, Repurchase Amount											$ 115,100,000
Convertible debt, unamortized discount											$ 35,700,000
Amortization of debt discount			500,000	1,800,000
Debt Instrument, Interest Rate, Effective Percentage											8.89%
Debt Issuance Costs, Gross											$ 900,000
Debt Instrument, Face Amount											$ 149,500,000.0
Cash convertible note hedge transaction				$ 20,500,000
Class of Warrant, Exercise Price of Warrants or Rights | $ / shares											$ 28.72
2027 Notes [Member] | Senior Notes
Components of short-term borrowings and other debt
Interest rate									7.35%
Debt Issuance Costs, Gross									$ 5,100,000
Debt Instrument, Face Amount								$ 18,000,000.0	$ 120,000,000
Interest Expense, Debt, Excluding Amortization			2,500,000
Line of Credit [Member]
Components of short-term borrowings and other debt
Interest rate									1.00%
Collateral Reinsurance Agreement
Future minimum lease payments for capital lease obligations
Restricted Cash and Cash Equivalents			$ 8,900,000				$ 11,800,000

[1] Amount necessary to reduce net minimum payments to present value calculated at the Company's implicit rate at inception. This amount also includes capitalized debt costs and the unamortized discount on the convertible debt.